1. Field of the Invention
The present invention relates to a light source unit emitting a laser beam for use in a laser beam printer, a laser facsimile apparatus, a laser image scanner for printing, a bar code reader, a copying apparatus or a sensor.
2. Description of the Prior Art
Light source units having semiconductor lasers are used as light emitting sources for various apparatuses such as the above-mentioned ones. For example, in an image forming apparatus such as a laser beam printer, a copying apparatus and a facsimile apparatus, a light source unit is used as an apparatus for irradiating a laser beam for forming images such as letters on an electrophotographic drum at a high speed. In an image scanner, it is used as an apparatus for irradiating a laser beam for making master film for printing. In a bar code reader and various types of sensors, it is used as an apparatus for irradiating a laser beam for obtaining information by means of reflected beams from a bar code and an object.
A well-known typical system using a light source unit of above-mentioned type is the one in which a laser beam is generated by a semiconductor laser and a lens which serves as a collimator. The laser beam is reflected by a polygonal scanner to form an image on a predetermined surface.
In such a system where only one semiconductor laser is used, as the processing speed depends on the scanning speed, and increase in the processing speed is limited.
In consideration of this problem, in order to achieve a higher processing speed, a light source unit has been proposed, which comprises a plurality of beam emitting devices including a package provided with a beam emitting chip which emits one beam. An optical system is provided for converting the laser beam emitted from the package into a parallel beam. A fixing member is provided for fixing the plurality of beam emitting devices in a predetermined arrangement. In fabricating a light source unit of this type, however, it is not easy to adjust the positions of the light emitting devices so that the parallel beams are equally spaced.
Another well-known system is the one in which a He-Ne laser or an Ar laser is used and recording is performed onto a recording vacuum cylindrical roller which rotates or a plane recording member by turning on and off the laser beam by use of an acoustooptic modulator. In order to achieve a multi channel in such an apparatus, however, a highly-accurate beam splitter for splitting one laser beam into a plurality of beams and a multi-channel acoustooptic modulator for severally modulating the plurality of beams are required. As a result, the cost increases.
In order to overcome this problem, a light source unit has been proposed in which a plate with equally spaced pinholes are formed so as to correspond to a plurality of semiconductor lasers which is used for intercepting beams other than the beams passing through the pinholes so as to equally space the parallel beams. The parallel beams can be equally spaced with this light source unit because it is easier to form pinholes so as to be equally spaced in the plate than to fix the semiconductor lasers so as to be equally spaced.
That is, although it is difficult to equally space the semiconductor lasers, it is easy to form pinholes in the plate so as to be equally spaced. By using such a beam restricting plate where pinholes are formed, equally spaced beams can be obtained even if, for example, the semiconductor lasers are not equally spaced.
In a case in which pinholes are used as described above, however, it is difficult to equalize the optical power or light quantity of each laser beam. This problem will hereinafter be explained with reference to FIGS. 4 to 6.
The cross section of a laser beam flux emitted from a semiconductor laser is not circular but elliptical. On the plate where pinholes are formed, a far field pattern (hereinafter referred to as FFP) is formed which is longer in a direction vertical to a p-n junction plane of the laser chip.
The position relations between the pinhole and the FFP can be divided into two cases, one case where the center of the FFP coincides with the center of the pinhole and another case where the FFP is decentered relative to the pinhole. As shown in FIG. 4, this decentering of the FFP can be roughly divided into a case where an FFP 10 is decentered from the center of a pinhole 25 in a direction parallel to the p-n junction plane (i.e. along the minor axis of the ellipse) and a case where the FFP 10 is decentered from the center of the pinhole 25 in a direction perpendicular to the p-n junction plane (i.e. along the major axis of the ellipse).
The degree of variation in light quantity of a beam (hereinafter referred to as pinhole beam) having passed through the pinhole 25 due to the above-mentioned decentering of the FFP from the pinhole 25 was examined by the following method with a measuring system shown in FIG. 5.
A laser diode 30 was fixed on a Z stage, and a lens 35 was fixed on a Z stage, and a plate 27 having a circular pinhole 25 was fixed on the XY stage. A plate 22 having a circular pinhole 20 with a diameter of 100 .mu.m and a photodiode 40 were arranged at positions which are approximately one meter from the pinhole 25. The plate 22 and the photodiode 40 were fixed on an X stage, and were movable in one direction (direction of arrow A) which was perpendicular to an optical axis AX.
After the positions of the laser diode 30 and the lens 35 have been adjusted, respectively, a beam 5 was irradiated from the laser diode 30 to the pinhole 25. At this time, by moving the plate 27 in an X direction (FIG. 4) on the XY stage the pinhole 25 can be decentered, for example, from a position 1.sub.0 where the center of the pinhole 25 coincides with the center of FFP 10 to a position 1.sub.1. Similarly, by moving the plate 27 in a Y direction on the XY stage, the pinhole 25 can be decentered, for example, from the position 1.sub.0 to a position 1.sub.2. Peak powers of a pinhole beam measured by the photodiode 40 and beam diameters formed on the photodiode 40 when the position of the pinhole 25 was thus decentered are graphically shown in FIGS. 6 and 7, respectively. FIG. 6 shows measurement results corresponding to the decentering in the X direction. FIG. 7 shows measurement results corresponding to the decentering in the Y direction.
As is understood from the measurement results of FIGS. 6 and 7, the beam diameter hardly varied in either case; however, in the case of the decentering in a direction parallel to the p-n junction (FIG. 6), the peak power changed even by a slight decentering compared to the case of the decentering in a direction vertical to the p-n junction (FIG. 7).
In the conventional light source unit having a plate where pinholes are formed at equal spaces so as to correspond to the semiconductor lasers, since the p-n junction plane of each light emitting means is arranged in a random direction, according to the measurement result, a slight decentering in any direction largely changes the light quantity of some of the laser beams.
Thus, in this prior art, although it is possible to equally space the parallel beams, if the positioning accuracy of the semiconductor laser is low, it is extremely difficult to equalize the light quantity of each laser beam.